Individuals with aspirin exacerbated respiratory disease (AERD) demonstrate selective airway hyperresponsiveness to leukotriene (LT)E4 by unclear mechanisms. This Project tests the role of a novel LTE4 receptor, GPR99, in mediating the potent effects of LTE4 on cutaneous and pulmonary vascular leak in mice, and in mediating Th2-dominated pulmonary inflammaton in a model of dust mite-induced pulmonary pathology by a pathway involving platelets, P2Y12 receptors, and prostanoids. We found that mice lacking both CysLT1R and CysLT2R (Cysltr1/Cysltr2-/- mice) show markedly increased and prolonged vascular permeability responses to intradermally administered LTE4 relative to wild-type (WT) controls, as well as an enhanced pulmonary vascular leak in response to LTE4. Cysltr1/Cysltr2-/- mice also show cutaneous and pulmonary vascular leak to LTC4 and LTD4 that are equivalent to those in WT mice. Moreover, Cysltr1/Cysltr2-/- mice show a markedly enhanced and prolonged vascular permeability response in IgE-dependent PCA, indicating that CysLTER mediates ear swelling occurring in response to the cysteinyl leukotrienes (cys-LTs) released by activated mast cells. Finally, intranasal sensitization and challenge with house dust mite extract elicits substantial cys-LT-dependent pulmonary inflammation in Cysltr1/Cysltr2-/- mice, implying that cys-LT-driven immune cell functions that lead to a Th2 cytokine profile and pulmonary eosinophilia can occur independently of CysLT1R or CysLT2R. We have identified GPR99 as a candidate receptor for LTE4 based on preliminary in vitro studies. Aim 1 is to further characterize GPR99 in vitro and to generate the deficient strains needed to prove that GPR99 is the LTE4-specific receptor that mediates vascular and cellular innate and adaptive immune responses in mice lacking CysLT1R and CysLT2R. Aim 2 is to determine the contribution of GPR99 to the cys-LT-dependent mast cell-mediated vascular leakage in the skin of Cysltr1/Cysltr2-/- mice. Aim 3 is to determine the contribution of GPR99 to the cys-LT-dependent allergen-induced Th2 cell mediated pulmonary eosinophilic inflammation in Cysltr1/Cysltr2-/- mice. We will also determine the expression of GPR99 in cells from individuals with AERD and aspirin-tolerant controls.